Environmental Code of Practice for integrated steel mills: section 3

Section 3: Environmental Concerns

The major activities and processes of relevance to this Code of Practice and the associated environmental releases are illustrated in Figures 2.1, 2.2, and 2.3.

3.1 Raw Materials Handling and Storage

The main environmental issue relating to raw materials handling and storage is the fugitive emission of particulate material arising from material transfers, truck traffic, and wind erosion of raw material storage piles. A secondary issue is the suspended solids and, in some cases, oil, contained in the runoff water from the storage areas.

Fugitive emissions of particulate are usually controlled by spraying stockpiles with water or crusting agents and ensuring that roadways and vehicle wheels are kept clean. The water runoff is usually directed to a wastewater treatment plant.

3.2 Cokemaking

Coke oven emissions may be intermittent or continuous. The combustion stack emissions resulting from the under-firing process are continuous and include CO, CO2, oxides of sulphur (SOx), NOx, and particulate matter. The levels of each will depend on the fuel used and the performance of the combustion control system. Intermittent emissions arise from a multitude of sources including the oven charging operation, coke pushing, transport and quenching operations, and process leaks from coke oven doors, topside lids, offtakes, and collector mains. Intermittent emissions include particulate and a wide range of hydrocarbons including benzene and PAH. Coke ovens are the major source of PAH emissions in the steel manufacturing sector and one of the major sources of PAH emissions from Canadian industry.

Potential sources of benzene emissions include leaks from process pumps, valves, vents, storage tanks, and associated equipment and the handling of light oil (a mixture of benzene, toluene, xylene, and other hydrocarbons) recovered from the COG. Cooling towers used for process cooling water may also be a significant source of benzene emissions if there are leaks in the heat exchangers. The coke by-products plants are the major source of benzene emissions in the steel manufacturing sector and one of the major sources of benzene emissions from Canadian industry.

3.3 Sintering

Emissions from the sintering process arise primarily from materials-handling operations, which result in airborne dust, and from the combustion reaction on the strand. Combustion gases from the latter source contain dust entrained directly from the strand along with products of combustion such as CO, CO2, SOx, NOx, and particulate matter. The concentrations of these substances vary with the quality of the fuel and raw materials used and combustion conditions. Atmospheric emissions also include volatile organic compounds (VOCs) formed from volatile material in the coke breeze, oily mill scale, etc., and dioxins and furans, formed from organic material under certain operating conditions. Metals are volatilized from the raw materials used, and acid vapours are formed from the halides present in the raw materials.

Combustion gases are most often cleaned in electrostatic precipitators (ESPs), which significantly reduce dust emissions but have minimal effect on the gaseous emissions. Water scrubbers, which are sometimes used for sinter plants, may have lower particulate collection efficiency than ESPs but higher collection efficiency for gaseous emissions. Significant amounts of oil in the raw material feed may create explosive conditions in the ESP. Sinter crushing and screening emissions are usually controlled by ESPs or fabric filters. Wastewater discharges, including runoff from the materials storage areas, are treated in a wastewater treatment plant that may also be used to treat blast furnace wastewater.

Solid wastes include refractories and sludge generated by the treatment of emission control system water in cases where a wet emission control system is used. Undersize sinter is recycled to the sinter strand.

3.4 Ironmaking

The main emissions from the blast furnace occur during tapping operations and are primarily iron oxide particulates. These are usually controlled by local hooding within the cast house to direct the emissions to a fabric filter. Variable quantities of hydrogen sulphide and sulphur dioxide are emitted from slag cooling and treatment. The control of these emissions is usually through process change or operating practices. Some emissions, including particulate matter, sulphur oxides, and other gases, are generated on an intermittent basis when de-sulphurization is practised. Emission control is usually by fabric filter. Some fugitive emissions, including iron oxides and graphite flakes, occur during hot metal transport to the steel melt shop.

Wastewater effluents arise from blast furnace gas cleaning and slag cooling and processing operations. Recirculation is used and the bleed stream is treated to remove solids, metals, and oil prior to discharge.

Slag is the main solid by-product. It can be processed in a variety of ways including granulating and pelletizing or cooled, crushed, and screened. The slag is sold as a by-product, primarily to the cement and construction industries. Sludge from the gas cleaning system may be recycled to a sinter plant or sent to a solid waste disposal site.

3.5 Steelmaking

Primary emissions of gas and particulate matter are collected in a hood above the mouth of the basic oxygen furnace (BOF) during oxygen blowing. These emissions include carbon monoxide, iron oxides, and other metal oxides. Fugitive emissions emerge from the mouth of the BOF during oxygen blowing but are minimized in modern plants by the use of a close-fitting hood. The primary emissions are usually controlled by a wet scrubbing system, although a few plants, including Stelco Inc., Hilton Works, use an electrostatic precipitator.

The fugitive emissions from hot metal transfer, scrap charging, oxygen blowing, tapping, and slag handling are usually collected by local hooding and cleaned in fabric filters.

Minor emissions of particulates arise from ladle metallurgy processes and vacuum degassing. These are usually collected and cleaned by fabric filters. Some wastewater effluent may be generated by the degassing process and is treated with other wastewater effluents.

The wastewater effluent from gas scrubbing is recycled and the bleed stream is treated to remove suspended solids and oil and to control pH.

The main solid wastes include steel skulls, slag, and waste refractories. Other solid wastes include the wastewater treatment sludge and dust from dry dust collectors. The steel skulls are recycled, the slag is crushed and screened for recycle or sale, and other solid wastes are recycled, where appropriate, or disposed to a landfill site.

3.6 Continuous Casting

Air emissions of particulate matter and metals arise from the transfer of molten steel to the mould and from the cutting to length of the product by oxy-fuel torches.

Wastewater effluents are generated during the cooling of the hot metal and include scale particles and oil.

Solid waste is generated from the cutting of the steel but is minor in amount and is recycled within the plant.

3.7 Hot Forming

Air emissions from hot forming include gases generated by the combustion of fuel in the heating furnaces and VOCs from rolling and lubrication oils.

Wastewater effluents are generated from the high-pressure water descaling of the hot steel and include suspended solids, oil, and grease.

Solid waste is primarily waste iron oxides recovered from the descaling and wastewater treatment operations and includes oil and grease.

3.8 Cold Forming

Air emissions from cold forming are primarily VOCs from rolling and lubrication oils. Some minor emissions result from the combustion of annealing furnace fuel.

Wastewater effluents are generated from rolling oil-filtering systems, leaks, and spills and include oil and minor amounts of suspended solids.

3.9 Pickling and Cleaning

The major air emissions are acid aerosols from the acid pickling operations and the acid regeneration plant, if acid regeneration is used.

The major sources of wastewater effluents are the acid pickling rinse water and acid fume scrubber, acid regeneration plant scrubber, and alkaline cleaning. Acid pickling rinse water discharges can be minimized by counter flow cascading and, in some cases, recycling to the acid regeneration plant. The wastewater effluents contain suspended solids, oil and grease, metals and acids. Wastewater effluent from alkaline cleaning is treated in a wastewater treatment facility.

The major sources of solid wastes are iron oxide from the acid regeneration process and sludge from wastewater treatment facilities.

3.10 Coating

Emissions from the coating process include VOCs (solvents), metal fumes, acid aerosols (electrolytic coating), particulate, and combustion products. These emissions are usually controlled by local hooding and fabric filters or wet scrubbers.

Wastewater effluents include wet scrubber discharge and electrolytic coating process wastewater and rinse water. These effluents contain suspended solids, metals, and acids and are treated in a wastewater treatment plant before discharge. Wastewater effluent and rinse water that contain Cr6 from chromium coating facilities are treated by an ion exchange process, and the chromic acid is recycled.

Solid wastes include zinc dross, tin oxide, tank sludges, and water treatment sludges. The zinc dross and tin oxide are sold, and other solid wastes disposed to landfill.

3.11 Environmental Release Inventories

The National Pollutant Release Inventory (NPRI) is a federal government regulatory initiative designed to collect annual, comprehensive, national data on releases to air, water, and land, and transfers for disposal or recycling of specified substances. The NPRI data support a wide range of environmental initiatives, including toxic substance assessment and pollution prevention and abatement. NPRI data are accessible by the public and provide information on all sectors - industrial, government, commercial, and others.

The Accelerated Reduction/Elimination of Toxics (ARET) is an initiative dedicated to decreasing the adverse effects of toxic substances on human health and the environment. ARET especially targets toxic substances that persist in the environment and bioaccumulate in living organisms. Through voluntary action, organizations that use, generate, or release toxic substances strive to reduce or eliminate their emissions of these substances. According to the 1998 ARET report, 14 of the 17 steel plants participated in ARET for the 1997 calendar year. The substances reported by integrated mills in 1997 are identified in the ARET list of substances.

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